This patent application is related to copending U.S. patent application Ser. No. 09/676,220 to John Frus and Michael Cochran, entitled Method And Apparatus For Generating A Sustained Arc At A Sparking Device, filed Sep. 29, 2000, which is hereby incorporated by reference in its entirety.
This invention relates generally to improving the performance of high-voltage flyback-converters, and more specifically to a circuit configuration that offers higher output power while maintaining high-efficiency and high-reliability. This invention improves the performance of capacitive-discharge ignition systems, and is broadly applicable to other systems where an energy storage capacitor must be charged to a high voltage, such as the system used to provide the power in pulsed-plasma thrusters for spacecraft propulsion.
Flyback-type DC-to-DC converters are well known in the art. They operate by allowing a current to increase in the primary of a transformer, and then abruptly terminating that current. The magnetic energy stored in the transformer at that instant is transferred to a secondary winding, producing a current and voltage that drives a load. This second step in the process is called the flyback cycle. An important property of flyback converters is that their output power is provided as a series of pulses, and the output during each pulse behaves as a current source and delivers a fixed quantity of energy. Flyback converters are very good at charging capacitors to store energy. First, they are able to deliver their output current pulses into a short circuit, which is similar to the load initially presented by an uncharged capacitor. Second, as the capacitor charges, they are capable of delivering a very high voltage, since their output voltage is not restricted by the turns-ratio of the transformer, as is the case for forward converters.
Pulsed-plasma thrusters used on spacecraft for positioning require periodic charging of an energy storage capacitor that supplies voltage to a pair of electrodes that are part of the thruster. Between these electrodes is a material such as Teflon that can be ablated (vaporized) by an electric arc across its surface. No arc occurs because the vacuum of space prevents arc formation. The arc is initiated by creating a small spark from a discharge initiation device, which is similar to a small sparkplug. Ions from the small spark bridge the gap between the main electrodes allowing large plasma to form along their trail. The energy storage capacitor discharges instantly, which creates a large current on the order of tens of thousands of Amperes. In order to propel the spacecraft, many pulses must occur repetitively since each pulse represents a very small thrust. Converters for such a system must have sufficient power to charge the capacitor quickly to a high voltage, and at a high repetition rate. An example would be a converter that stores 50 Joules of energy in the capacitor three times per second; this converter needs a throughput of 50Jxc3x973/sec=150 Watts. Also important is that it must be extremely reliable, small in volume, and lightweight. A converter made according to this invention offers these advantages.
Capacitor Discharge ignition systems (CD ignition) is common in reciprocating engines, turbine engines, and stationary ignition applications such as burners. In a CD ignition, a converter must charge an energy storage capacitor that is discharged by a switch or semiconductor-device to create a spark to ignite a fuel mixture. In turbine engines, spark rates are generally low but energy per spark can be very high (tens of Joules at ones of sparks/second). In a reciprocating (piston) engine, much lower energy may work, but a high spark rate is required in multiple cylinders (tens of millijoules at hundreds of sparks/second). The latter application particularly benefits from a converter built according to this invention because of its ability to deliver the higher wattage required when the engine is run at high RPM.
In these demanding applications there are several characteristics that limit performance of flyback converters. First, they are power-limited. These converters are prevalent in low power applications, typically tens of Watts. They are not as good for higher power because their input current generally increases to allow more energy storage during each charge cycle. This leads to higher I2R losses (current squaredxc3x97resistance) in the transformer windings and efficiency suffers. Second, the operating frequency may become a limiting factor due to the time it takes to charge the primary or discharge the secondary at higher power levels. At this limited frequency, output ripple (peaks and valleys in the output voltage) is severe. Fundamentally, flyback converters have an output that resembles half-wave AC rectification. Power delivery only occurs during one half-cycle, and is zero during the intervening half-cycles. Large input and output filter components must be used to smooth both the power input and the power delivery, which adversely affect system weight and size, reducing their desirability for use on spacecraft or other airborne applications.
In order to overcome these limitations, it is an object of this invention to deliver higher output-power without incurring high electrical losses or weight penalties by providing interleaved operation of two or more related flyback circuits that share a common load.
It is a related object of the invention to more quickly charge a load capacitor to a high voltage. It is another related object of the invention to allow higher rates of periodic discharge of the load for pulsed applications in spacecraft thrusters or ignition applications.
It is another object of the invention to deliver steady low-ripple output power without requiring larger output filter components.
It is a further object of the invention to provide at least partial redundancy so that failure of certain key power electronic components will only reduce, but not eliminate power output.
It is a further object of the invention to provide improved control of charging and regulation of load voltage and load current.
It is a further object of the invention to reduce the input current ripple in high power applications thus reducing the size of input filter components. It is a related object of the invention to reduce electromagnetic interference emanating from either input or output connections.